How to create recesses in a curved surface?
Anything that has to be cut into a curved surface, I tend to build flat and curve into shape later. Using this method I have a model of one 'L' section and a bezier circle (origins of both objects matching each other):



Using an Array modifier with a count of '3', followed by a Subsurf set to '2' subdivisions and finally a Curve modifier:



I get this result:



Using modifiers allows you to easily manipulate the original non-curved section, while also seeing how it will look when curved into a circle.

Once you are happy, modifiers can be applied and the two ends of the circle can be merged (either by W> Remove Doubles, if the vertices are close enough, or more manual merging if they are not).

Also note that increasing the resolution of the curve on the curve's 'Data' tab will improve results. This example had a curve resolution of 30.